Oleuropein Modulates ESR2/ERβ-Associated microRNA Responses and Alters 5-Fluorouracil Activity in the Ehrlich Ascites Tumor Model: An In Vivo and Ensemble Docking Study

Oleuropein (OL) is widely recognized for its anticancer potential, yet its impact on chemotherapy response remains insufficiently defined. This study investigated whether OL modifies the efficacy of 5-fluorouracil (5-FU) and whether these treatment responses are accompanied by changes in ESR2/ERβ expression and associated miRNA profiles in the Ehrlich ascites tumor model. OL (150 mg/kg) was administered at different treatment schedules, alone or with 5-FU (20 mg/kg), and tumor burden, ascites formation, ESR2 mRNA, ERβ protein, miR-146a-5p and miR-155-5p expression were evaluated together with validated ERβ ensemble docking. While 5-FU markedly reduced viable tumor-cell burden, concomitant OL treatment substantially weakened this effect and restored tumor-cell numbers toward untreated levels. In contrast, OL administered at tumor initiation or prophylactically reduced ascites accumulation and produced distinct ESR2/ERβ expression profiles. OL also markedly altered chemotherapy-associated microRNA responses, suppressing the strong 5-FU-induced increase in miR-146a-5p and reducing miR-155-5p after prolonged exposure. Native-ligand redocking confirmed high reliability of the ERβ docking protocol (RMSD 0.242–0.411 Å). These findings identify a treatment-dependent interplay between OL, ERβ-associated responses, and 5-FU response, revealing that OL may exert both favorable and potentially antagonistic effects depending on therapeutic context.